1. Field of the Invention
The present invention relates to a suspension insulator for high voltage transmission, which improves pollution withstand voltage characteristics.
2. Description of Related Art
In the suspension insulator, if a salt component in seawater carried by wind and weather or a salt component in an exhaust gas from a factory is adhered to a surface of the suspension insulator and the suspension insulator becomes a wet state due to such a salt component, an insulation resistance of a surface of the suspension insulator is decreased. In this case, there is a drawback such that a flashover may occur due to reduction of pollution withstand voltage characteristics. Therefore, in the case of performing an insulation design of power transmission equipment, it is very important to take the pollution withstand voltage characteristics of the suspension insulator into consideration.
Generally, it is known that the pollution withstand voltage characteristics can be improved by designing a longer creepage distance of the suspension insulator. Therefore, in the known suspension insulator, a method for designing a longer creepage distance by making a diameter of shed of the suspension insulator larger or by increasing the number of ribs arranged on an under surface of the suspension insulator or a depth between the ribs is adopted. However, in the method mentioned above, there is a drawback such that a dimension of the suspension insulator becomes necessarily larger.
Moreover, except for a geometrical shape design mentioned above, there is known a method such that the pollution withstand voltage characteristics are recovered by arranging a conductive glaze on a surface of the insulator so as to flow a weak current thereon and by drying wet pollution substances adhered to the surface of the insulator by means of a heating effect due to the weak current. Further, a method for improving the pollution withstand voltage characteristics is also actually utilized wherein an electric field relaxation is performed by relaxing a potential inclination along the surface of the insulator and a discharge due to a partly and rapidly deviating electric field. This method mentioned above is very effective, but there arises a deterioration such that a surface of the conductive glaze is sometimes eroded by an environmental pollution or such that a surface resistance is sometimes increased.
In order to solve the drawbacks mentioned above, the present applicant proposed a suspension insulator, in Japanese Patent Application No. 11-369186, such that a circular electrode is arranged on an under surface, to which a glaze is arranged, axially with respect to a pin fitting, or, such that a resistance zone having an electric resistance of 1-100 Mxcexa9 (corresponding to a surface resistance of 4.3-430 Mxcexa9) is arranged on the circular electrode and on a portion between the circular electrode and the pin fitting. The suspension insulator mentioned above shows a sufficient performance for the pollution withstand voltage characteristics that are necessary at that time. However, the suspension insulator mentioned above sometimes shows a poor performance for higher pollution withstand voltage characteristics and a higher discharge suppress performance that are recently required. Therefore, a suspension insulator having higher pollution withstand voltage characteristics and a higher discharge suppress performance is required.
An object of the present invention is to eliminate the drawbacks mentioned above and to provide a suspension insulator which can respectively perform an improvement of pollution withstand voltage characteristics and a prevention of a discharge and which can improve a voltage distribution along an insulator string.
According to a first aspect of the invention, a suspension insulator having a shed including an upper surface and an under surface comprises: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; a resistance zone having a surface resistance (1 cmxc3x971 cm) of not greater than 4xcexa9 and arranged on the under surface at an inner portion continuing from the pin fitting; and a conductive zone arranged on the under surface at an outer peripheral portion of the resistance zone.
Moreover, according to a second aspect of the invention, a suspension insulator having a shed including an upper surface and an under surface comprises: a pin fitting arranged at its center portion and projected from the under surface; a plurality of circular ribs arranged around the pin fitting in a concentric manner; and a resistance zone arranged on the under surface at an inner portion continuing from the pin fitting and existing between the pin fitting and an inner root portion of the rib.
As mentioned above, since the resistance zone having a predetermined resistance and the conductive zone are arranged in the first aspect of the invention, or, since the resistance zone is arranged at a predetermined position in the second aspect of the invention, a position to which a high electric power is concentrated can be moved outward with respect to the center of insulator, and thus it is possible to form a stable dry zone with a low power. As a result, it is also possible to improve a voltage distribution along the insulator string and thus it is also possible improve pollution withstand voltage characteristics and to prevent a discharge. In the present invention, a term xe2x80x9ca portion between the pin fitting and an inner root portion of the ribxe2x80x9d means a portion between the pin fitting and a bottom of the target circular rib that is near to the pin fitting among a plurality of circular ribs.
As a preferred embodiment of the invention, both in the first aspect of the invention in which the resistance zone and the conductive zone are arranged and in the second aspect of the invention in which the resistance zone is arranged at a predetermined position, the following embodiments are preferred since the effects of the invention can be further improved: a material of the resistance zone is a conductive glaze of ferric oxide series or a conductive glaze of tin oxide series; and a resistance zone as is the same as the resistance zone arranged on the under surface at an inner portion continuing from a cap fitting arranged on a head portion of the suspension insulator is arranged on an upper surface thereof, and a conductive zone as is the same as the conductive zone arranged on the under surface is arranged on the upper surface at an outer peripheral portion of the resistance zone. Moreover, in the first aspect of the invention, the following embodiments are preferred since the effects of the invention can be further improved: the conductive zone has a half or less surface resistance as compared with that of the resistance zone; the resistance zone is arranged at a portion between the pin fitting and an inner root portion of the rib; a material of the conductive zone is a metal, a conductive glaze of ferric oxide series having a low resistance or a conductive glaze of tin oxide series having a low resistance, and a thickness of the glaze of the conductive zone is larger than that of the glaze of the resistance zone; and the conductive zone, whose thickness is larger than that of the resistance zone, is formed by arranging a recess to the under surface at a portion to which the conductive zone is formed, and filling a conductive material such as the conductive glaze of ferric oxide series or tin oxide series in the thus formed recess. Furthermore, in the second aspect of the invention, the following embodiment is preferred since the effects of the invention can be improved: the resistance zone has a surface resistance (1 cmxc3x971 cm) of not greater than 4Mxcexa9.